Researchers compared the anatomy and behavior of dogs to wolves over thousands of years and found that muscles in the face were similar except for one small thing. Unlike wolves, dogs have a very small muscle that allows them to dramatically raise their inner eyebrow.

The researchers suggest that when dogs raise their inner eyebrow, it prompts a nurturing response in humans because it makes the dogs' eyes seem larger and more infant like. It also mimics the expressions humans make when they're sad.

"The findings suggest that expressive eyebrows in dogs may be a result of humans' unconscious preferences that influenced selection during domestication," said lead researcher and comparative psychologist Juliane Kaminski at the University of Portsmouth, in a statement. "When dogs make the movement, it seems to elicit a strong desire in humans to look after them. This would give dogs, that move their eyebrows more, a selection advantage over others and reinforce the 'puppy dog eyes' trait for future generations."

The study research team included behavioral and anatomical experts in the U.S. and U.K. and was published in the journal PNAS.

Co-author and anatomist Anne Burrows of Duquesne University said the evolution of the eyebrow muscle changes was "remarkably fast" and "can be directly linked to dogs' enhanced social interaction with humans."

Evolutionary tweaks fuel adoptions

The team's previous research shows that dogs raise their eyebrows more when people are looking at them than when they're not.

For the study published in PLOS One, researchers observed 27 shelter dogs and counted the number of times each animal raised its inner brows and widened its eyes when a person approached. The dogs were all Staffordshire bull terriers and mastiffs between the ages of 7 months and 8 years old, and the ones that raised their brows were consistently adopted faster than those that didn't.

"The results of this research suggest that wolves which produced child-like expressions may have been more tolerated by humans, and so modern dogs have inherited these features," said head researcher, evolutionary psychologist Bridget Waller.

"We might have automatically opted for dogs which produced facial movements that enhanced their baby-like faces. Raised inner brows are also closely associated with sadness in humans and so another possibility is that humans are responding to a perceived sadness in the dog."

Previous research suggested that the domestication of wolves was simply a byproduct of people avoiding aggressive animals. However, these new studies indicate that dogs' child-like expressions are a result of indirect selection by humans.

Editor's note: This story was originally published in January 2014 and has been updated with new information.

Mary Jo DiLonardo covers a wide range of topics focused on nature, health, science and anything that helps make the world a better place.